At present, the PCI (Peripheral Component Interconnect) Express specification is being widely adopted as an interface bus specification after the PCI specification for connecting various kinds of component elements that constitute a computer and various kinds of peripheral devices. With the PCI Express specification unlike the traditional PCI specification, a serial interface that realizes sequential data transmission using point-to-point interconnection between devices is implemented.
The smallest unit for a transmission path used in the PCI Express 1.1 specification is called a “lane”, through which the full duplex transmission of 2.5 Gbps in simplex mode and 5.0 Gbps in duplex mode is possible. In order to transmit 8-bit data, the data needs to be expanded by two bits for clock recovery and converted into 10-bit data, and therefore, an effective data transfer rate is 2.0 Gbps (250 MB/s) in simplex mode and 4.0 Gbps (500 MB/s) in duplex mode.
An actually used socket compliant with the PCI Express specification, into which a detachable adapter card is inserted, is often constituted from more than one lane. A PCI Express socket that is constituted from a single lane is called a “PCI Express ×1”, and a PCI Express socket that is constituted from n lanes is called a “PCI Express ×n”. Today, products with sockets of ×1, ×4, ×8, ×16, and the like are available. While the data transfer rate with the PCI Express 1.1 specification in simplex mode is up to 4 GB/s for PCI Express ×16 and 8 GB/s for ×2, PCI Express 2.0 with which transmission speed per lane is doubled has been recently released, and further improvement in performance is expected in the future.
The PCI Express configuration has successfully decreased the number of connecting terminals at the same transfer rate as that of the PCI by adopting a high-speed serial transmission method, and has realized a higher transfer rate by increasing the number of lanes. Further, the PCI Express has flexibility in that data can be correctly transferred between a socket and an adapter card if the number of lanes for the socket is greater than, even if not equal to, that for the adapter card.